1. Field of the Invention
The present invention relates to check valves of the type which automatically close in response to a flow reversal in a liquid flow passage. The invention relates particularly to means for suppressing pressure surges which occur in the flow passage as a result of sudden closure of the check valve. While the present invention has application to various types of check valves for any desired application, the invention is described in connection with check valves of the type utilized in nuclear steam generator plants.
2. Description of the Prior Art
In a typical nuclear steam generating plant, the cooling water for a nuclear reactor is circulated through a steam generator, wherein the water from the nuclear reactor is circulated in heat-exchange relationship with respect to a secondary feedwater. The feedwater is heated and converted to steam, which is then directed through an associated steam turbine to a condenser where the steam is converted back to water, the condensate being pumped back to the steam generator as the feedwater. The discharge from several of the steam generators may be coupled between the steam generators and the associated steam turbine in a manifolded arrangement. Typically, the turbine, the condenser and the associated pumping equipment are located some distance from the steam generators. In the event of an earthquake or other severe shock, the long extent of conduit between the pumping equipment and the steam generator may be susceptible to damage or rupture. Accordingly, a check valve is typically provided in this conduit adjacent to the steam generator. In the event of a break in the conduit upstream of this check valve, the sudden drop in pressure at the break would create a large pressure differential which would result in the pressurized feedwater in the steam generator flowing in the reverse direction through the check valve toward the break. The check valve responds to this reverse flow by immediately closing.
One difficulty with prior check valves is that upon sudden closure they generate a pressure surge or "water hammer" effect which is reflected back and forth between the steam generator and the check valve. This pressure surge can be as great as six times the normal line pressure, which is typically about 1,300 psi. These very high pressure surges can damage the steam generator tubes or instruments in the line.
It is known that providing damping on the check valve minimizes the "water hammer" effect. In other words, the closure of the check valve is slowed down to prevent it from slamming shut. This technique works quite well in alleviating the pressure surge, but it necessarily results in a much greater loss of feedwater from the steam generator through the break, since the valve is open longer. This can be a serious drawback in nuclear steam generator applications, since the feedwater in the steam generator serves as a coolant for the reactor cooling water and substantial loss of this coolant can result in increased heating of the reactor core. While the steam generator is typically provided with alternative feed lines, designed to supply feedwater in the event of a break in the main feed line, these alternate lines are of much smaller diameter and, therefore, cannot make up lost steam generator feedwater very rapidly.